In the case of hybrid systems where the electrical machine such as Permanent Magnet Synchronous Motor (PMSM) is always connected to the Internal Combustion engine (IC engine), the PMSM is bound to rotate whenever the IC engine is functioning, even when the electrical machine is not intended to develop torque. If there is a failure/fault identified in the inverter/electrical machine or other components of the hybrid systems, the inverter has to be operated in safe state comprising an active Short Circuit (SC) or Freewheel (FW) state.
When the inverter circuit is driven in the SC state, there is definite amount of no load losses in the PMSM. But on the other hand, being in SC state ensures the galvanic isolation between the battery and the inverter circuit In FW state, the inverter circuit acts like a three phase rectifier due to the anti-parallel diode of switch modules. So when the rectified voltage from inverter circuit is more than the battery voltage, a current flows from the PMSM to the battery through the inverter circuit.
However, if the rectified voltage from the inverter circuit is less than the battery voltage, no current flows from the PMSM back to the battery. Being in FW state does not necessary guarantee electrical isolation between the battery and the inverter circuit. Also, if there is current flow from the inverter circuit to the battery in FW state, a certain amount of power is developed in the PMSM which is used for charging the battery. If the rectified voltage from the inverter circuit is less that the battery voltage, no current flows from the inverter circuit to the battery, and hence there is galvanic isolation between the inverter circuit and the battery, and the losses are minimum.
As a safety measure it is usual practice to make inverter gate driver state as SC when a failure is identified, due to the obvious reason that SC state ensures electrical isolation between High Voltage (HV) battery and inverter. In the case of vehicles which have high torque requirements (like truck and other commercial vehicles) the electrical machine will have high no load losses due to the higher value of the rotor flux. The electrical machine that are needed to develop high torque are designed with high value of rotor flux in order to develop more torque. The higher value of rotor flux causes high no load losses. The no load losses consist of copper loss, eddy current loss and hysteresis loss mainly. Iron loss (Eddy current and hysteresis loss) are proportional to rotor flux. The copper loss (I2R) depends on the current in the machine (High during SC mode operation)
Further, the commercial vehicles usually has long travels without stops. Having the inverter circuit in SC state for a long time causes the electrical machine to heat up as there is high current flowing through its winding. This rise in temperature can potentially damage certain parts in the stator and finally leading to damage of the electrical machine itself. Since, the electrical machine is directly coupled to the IC engine (as in the case of mild hybrids) the machine cannot be isolated from the shaft and so it is bound to rotate along with the IC engine which develops the torque now.
Hence, in brief, whenever a failure is encountered in the hybrid system, the inverter is forced to operate in the SC state, which increases the no load losses and temperature of the electrical machine. But, remaining in SC state for a long time is not suitable since there will a temperature rise in the electrical machine due to no load losses. The electrical machine is likely to get damaged due to the no-load losses when the system is in SC state for a very long time in the vehicles where the electrical machine is directly coupled to the IC engine.
A patent literature DE102013226560 discloses a device and method for operating an electric machine. The invention results in improved switching from the idling mode to an active short circuit mode of an electric machine. The switch from the idling mode to the active electric short circuit mode is delayed until predefined voltage conditions have been reached on the external terminals of an electric machine or until the rotor of the electric machine is in a predetermined position corresponding to the required voltage conditions.
There is a need to provide an efficient method for deciding on which safe state the inverter should operate in case if inverter is requested to be in safe state, under a fault condition. Further, there is a need for minimizing the temperature rise and the negative torque in the electrical machine.